We study intracelluarly the physiology of single retinal neurons in cats and pigeons. Intensity response curves, input from rods and different cone types, receptive field properties, and response waveforms are assessed. Individual neurons, stained through the microelectrode with horseradish peroxidase (HRP), are identified according to anatomical schemes. Using the resolution of the electron microscope, we can identify synaptic contacts made between the stained cells, which have electron opaque profiles, and other retinal neurons such as photoreceptor, horizontal, bipolar, amacrine, and ganglion cells. The morphology of individual neurons in monkey, cat and pigeon retinas is also studied at the light microscopic level using Golgi impregnation. The size, shape, and orientation of pigeon amacrine cells and primate horizontal cells suggest their specific functional roles. In outer plexiform layer of primates, electron microscopy of Golgi stained horizontal cells reveals the contacts made with different classes of photoreceptors. Material fixed for ultrastructure demonstrates contacts between photoreceptor terminals and their variation in density and type with retinal location. Changes in the synaptic ultrastrcture of rods and cones are correlated with their physiological state of adaptation; activity of synapses in response to varying stimuli is assayed by uptake of HRP into synaptic vesicles.